Carbon nanotubes have advantageous mechanical, electrical, and thermal properties at the nanoscopic scale. Because of their unique structure, dimensions, and form factor (high length/diameter ratio), nanotubes are proving to be very promising in terms of applications. They can be used for replacing conductive paths in integrated circuits because of their low migration tendency, their high maximum currents, and other properties. Studies show that these nanostructures develop a very high tensile strength and that their electrical behavior can vary from semi-conductive to metallic depending on their structure. This electrical conductivity is accompanied by excellent thermal conductivity. Therefore the idea of incorporating them into composite materials quickly made an appearance.
The properties of currently manufactured composite materials already seem satisfactory for many applications. However, an improvement in the thermal and electrical conductivities as well as in the mechanical performance would expand their fields of application, particularly for airplane fuselages, disk brakes, and other such applications.
In the current context which favors the development of composite materials based on carbon fibers, true progress would be a solution resulting in obtaining on a macroscopic scale the properties of carbon nanotubes that appear on a nanoscopic scale. However, the mechanical and thermal performance obtained by employing carbon nanotubes as the conventional filler for composites is debatable, while the electrical conductivity properties obtained in composites with carbon nanotube fillers is satisfactory. To improve the effectiveness of incorporating carbon nanotubes into composite materials and ultimately improve the properties of the resulting material, several paths were considered. Two of the main ones are described below. It is in fact possible:                to align carbon nanotubes within a matrix of the composite (polymer for example),        to integrate carbon nanotubes into an existing reinforcement of the composite (a reinforcement such as fiberglass, alumina fiber, or carbon fiber for example).        
Carbon nanotubes can therefore be incorporated into the reinforcement or matrix system in an ordered or unordered manner and must ideally have cohesion with the reinforcement fibers.
Direct growth of nanotubes on carbon fibers has thus been considered. The carbon nanotubes obtained on the fibers may be unordered.
Another path would be to produce them so they are perpendicular to the axis of the fiber, the way they already are on flat substrates (of silicon, quartz, or other material) when certain growth methods are used. This last configuration, with carbon nanotubes oriented perpendicularly to the axis of the fibers, should result in an increase in the contact area between the reinforcement formed by the woven fibers and the surrounding polymer matrix, encouraging the transfer of thermomechanical properties, which potentially should increase the thermal conductivity properties of the composite. A method which could result in this type of material enabling such a transfer of properties on the macroscopic scale and which could be applied on the industrial scale, continuously or semi-continuously, would be of undeniable interest.
Growing carbon nanotubes on carbon substrates of any type (flat surface, fibers, foam, particles) is known to be problematic. Results obtained to date show growths of short and tangled carbon nanotubes on carbon surfaces, of a very low density, not at all comparable to the dense and aligned “carpets” obtained on quartz or silica type substrates. Some studies even mention an absence of growth. These growth problems are also encountered on other types of substrates besides carbon substrates, particularly metals (stainless steel, palladium, gold, etc.).
Growth on carbon substrates, such as carbon fibers, is generally achieved by chemical vapor deposition (CVD).
The CVD growth techniques employed are based on the breaking down of a carbon-containing gas by metal particles (referred to as “catalyst particles”), in a furnace. Two main types of methods can be distinguished:                the pre-impregnation method, in which the particles are created before the actual growing of the carbon nanotubes, and which is implemented by soaking in solutions of appropriate metallic salts, followed by an injection of carbon-containing gases for growing the carbon nanotubes,        the method of injecting and vaporizing in a furnace a solution of an organometallic compound in an organic solvent which is the carbon source.        
Some studies propose applying surface treatments to the fibers to encourage the growth of carbon nanotubes on their surface. Some of these methods have improved the tube growth (in terms of density, length, or other aspects) but the results remain far below the results obtained on substrates other than carbon or metal substrates.
A study by W. Z. Li et al, published in Chem. Phys. Lett., vol. 335, pages 141-149 (2001) was conducted on a flat substrate of graphite, coated by sputtering a stainless steel film (Fe:Cr:Ni=70:19:11), which, after annealing, led to the formation of stainless steel particles of a size that varied with the thickness of the film deposited on the surface of the graphite film. The carbon nanotubes were grown for an hour at a temperature of 660° C., using acetylene diluted in nitrogen. For a particle size of 40 nm, the scanning electron microscopy (SEM) images show a growth of not very dense carbon nanotubes, and nanotubes which are disordered horizontally to the substrate surface. The nanotubes seem to only form on particles that are spherical, and not on particles that have coalesced and have random shapes.
It seems that the results for Fe/Ni alloys on graphite are more encouraging than those for Fe or Ni alone, because under the same conditions, the carbon nanotube growth takes place on the alloy particles and not on the pure metal particles.
Studies conducted to date concerning the growth of carbon nanotubes on carbon fibers (of any type) include few conclusive studies using the CVD technique. Most of the protocols consist of impregnating the fibers before the CVD deposition, via the wet process, and reduction.
The impregnation is done by soaking fibers in a solution (aqueous or organic) of transition metal salts (iron, nickel, cobalt, or a mixture of these). The fibers are then heated in a reducing atmosphere to allow the formation of catalyst particles. Then the growth of carbon nanotubes is achieved by heating in a furnace into which a current of carbon-containing gas is directed. The resulting morphology is a thin sheath (a few micrometers) formed of carbon nanotubes around the carbon fiber. The carbon nanotubes are not very dense and are unordered surrounding the carbon fibers, and more or fewer carbon particles (sub-product of the synthesis reaction) are agglomerated around them.
Studies that do not use impregnation are inconclusive. One can cite patent FR-2 841 233 which shows a growth of nanotubes on carbon fibers that is not very dense, short in length, and without orientation, while carpets of dense and aligned carbon nanotubes are obtained on quartz or silicon substrates under the same growth conditions. The treatments mentioned in the literature for carbon fibers are most often oxidation treatments and consist primarily of chemical treatments (acids, hydrogen peroxide), and plasma treatments (corona or other) with variable atmospheres. Other types of treatments are mentioned in numerous publications, whether for functionalizing the fiber surface to improve the fiber-matrix interface, or encouraging the anchoring of nanotubes or chemical functions to the surface. Here again, the techniques discussed are generally chemical or physicochemical in nature (combining plasma and chemical deposition, for example silsesquioxane), or concern gamma irradiation.
The most common surface treatment involving deposition of sub-layers for this system consists of soaking in a solution of organosilicons and metal salts that are precursors of catalyst particles. This is hydrolyzed by adding acid to the bath. A coarse film, including the precursor metal salts primarily composed of silica (SiO2), then forms on the surface of the fibers. Once dried, the fibers covered in this manner are brought to a high temperature (typically 800° C.) in a reducing atmosphere (inert gas with several percentages of hydrogen added) in order to reduce the metal salts into catalyst particles. The resulting SiO2 film serves both as a diffusion barrier for the later growth of carbon nanotubes and as anchor sites for the catalyst particles (pre-impregnation techniques), as is described in patent FR-2 844 510. In general, a compound such as tetraethyl orthosilicate (TEOS) or 2(4-chlorosulfonylphenyl)ethyltrichlorosilane is used. Certain studies were able to show excellent nanotube anchoring by this method. The condition of the fiber surface after treatment based on organosilicons remains rough, however, having a micrometric thickness that is already fissured. It is also not very adherent, but the carbon nanotubes obtained form a continuous layer of tangled tubes that are at most several micrometers in length.
In patent application U.S. Ser. No. 11/523,731, a micrometric deposit of silicon carbide (SiC) is applied to the fibers before the carbon nanotubes are grown. This involves coating the fibers with a polymer precursor which is then polymerized at 200° C., then pyrolyzed at 1000° C. for an hour. The process enables the formation of a film of SiC that is several micrometers thick. The carbon nanotubes are grown on these coverings in a second step. The observed growth is dense and radial. However, the thickness of these SiC layers can pose problems concerning the mechanical properties of the composite formed.
The feasibility of growing carbon nanotubes on carbon fibers by aerosol-assisted CVD deposition, with the use of an organometallic precursor solution such as metallocene in a hydrocarbon, is explained in the document corresponding to published patent PCT/FR05/00201. Various surface treatments are suggested, particularly the application of nanometric layers of SiO2, SiO, SiC. Only the application method for SiO is described in detail, involving a phase of evaporating commercial SiO at 1100° C. However, the growth of carbon nanotubes during the second step of these processes remains sporadic and slow. The results illustrated remain far below the densities obtained on oxide substrates (Al2O3 fibers for example). The general orientation of the nanotubes is vertical, but in no case is this aligned with or perpendicular to the axis of the fiber.
In general, the following points are clear from the research mentioned above:                growing carbon nanotubes on carbon substrates is difficult,        the density of the carbon nanotubes obtained is generally low, and even very low, with no noticeable improvement by simple methods (such as synthesis parameters),        the length of the carbon nanotubes obtained is often small and difficult to control, with slow growth rates,        the nanotube morphology presents numerous defects (particularly curvature),        the arrangement of the carbon nanotubes is random or can tend towards a particular orientation without obtaining a growth of carbon nanotubes that are actually aligned.        
Current studies show that it is difficult to grow carbon nanotubes on fibers when these fibers were not previously treated. Pretreating the surface constitutes a prior step separate from the step of growing the carbon nanotubes, particularly in its use of a treatment method that is clearly different from the deposition method. Only with difficulty does such pretreatment lead to a noticeable improvement in growth.
In spite of adaptations to the processes or pretreatments employed, the studies show an inability to equal, in terms of density, alignment, or length, the growth of carbon nanotubes obtained on substrates such as quartz.